The unique properties which characterize organic compounds are due not solely to carbon atoms, but rather to the bonding of carbon atoms to each other and to the combination of carbon and hydrogen atoms. Silicon, in the same group of the periodic table as carbon, shares some of the bonding characteristics of carbon and forms analogous catenated structures but has its own unique qualities, namely, generally greater chemical reactivity, and has been the subject of extensive research in recent years.
Catenated silicon systems are known and are reviewed by R. West in G. Wilkinson, F. G. A. Stone, and E. W. Abel, "Comprehensive Organometallic Chemistry", Volume 2, Chapter 9.4, pages 365-387, Pergamon Press, New York (1982). The silicon-silicon bonds in such systems are most often formed from two silicon-halogen bonds with a Periodic Groups IA or Group IIA metal. Generally, groups such as alkyl, halogen, alkoxy, or aryl are predominantly attached to the catenated silicon backbone.
In contrast to the reported progress in the field of organo-substituted polysilanes, the syntheses of the silicon analogs of the parent carbon polymers such as polyethylene or polypropylene have remained more elusive. These polymers are classed as silicon hydrides, referred to in this patent application as polyhydridosilanes, and are generally reactive towards the atmosphere and not amenable to preparation and study without manipulation using vacuum line and/or controlled atmosphere (dry box) techniques.
A few examples of catenated silicon systems containing hydrogen atoms attached to a silicon backbone are known: cyclic polysilanes (SiH.sub.2).sub.n where n=5, 6 have been made in multistep syntheses by E. Hengge and G. Bauer, Angew. Chem. Intl. Ed. Eng., 12 (1973), 316 and E. Hengge and D. Kovar, Angew. Chem. Intl. Ed. Eng., 16 (1977), 403; mixtures of linear and branched polysilanes H(SiH.sub.2).sub.n H where n=2 to 8 have been made by the hydrolytic decomposition of magnesium silicide by K. Borer and C. S. G. Phillips, Proc. Chem. Soc., 189, (1959). Coupling of alkylhalohydridosilanes with metals to yield Si--Si bonds from systems containing Si--H and Si--Cl bonds and related reactions are reviewed by M. Kumada and K. Tamao in F. G. A. Stone and R. West "Advances in Organometallic Chemistry", Volume 6, pp. 34-36, Academic Press, NY (1968) and disclosed in U.S. Pat. No. 4,276,424.
Insoluble polysilanes have been disclosed. Aromatic silane polymers, formed in radio-frequency plasmas, and which are resistant to high temperature, adherent to substrates, and electrically insulating are disclosed in U.S. Pat. No. 4,397,722. Insoluble orange powders having the approximate composition (SiH.sub.n).sub.x where n is 1 or 2 and x is large are taught in UK Patent Application GB 2,077,710A. As is known in the art, a substantial degree of crosslinking will render a polymer insoluble (see Schultz, A. R., Encyclopedia of Polymer Science and Technology, 4, 336, John Wiley & Sons, NY (1966)).
Silyl derivatives of Periodic Group IA or IIA metals can be formed from silicon-halogen, silicon alkoxy, silicon-hydride or silicon-silicon bonds, as described in C. Eaborn, "Organosilicon Compounds", Chapter 12, pages 357-360, Academic Press, New York (1960). Reaction of catenated silicon systems with alkali metals to give delocalized radical anions has been reviewed by R. West in "Comprehensive Organometallic Chemistry", supra, pp. 393-395.
In the thermal treatment of polymers wherein crosslinking occurs, the resultant materials become stabilized due to the formation of a rigid insoluble network and the stable products are referred to as pyropolymers by S. D. Bruck and P. F. Liao (J. Polymer Sci., Part A-1, 8, 771 (1970)). Pyropolymers have also been described as materials that have intermediate properties between polymer and carbon by G. M. Jenkins and K. Kawamura, "Polymeric Carbons--Carbon Fibre, Glass and Char", p. 1, Cambridge University Press, London, England (1976).
Formation of elemental silicon by the energetic decomposition of gaseous molecules containing hydrogen and/or halogen and from one to about three silicon atoms is well known and is taught, for example, in U.S. Pat. Nos. 4,363,828 and 4,202,928.
Polymeric precursors to elemental silicon are less well known, although perhalopolysilanes are mentioned as precursors in U.S. Pat. Nos. 4,374,182 and 4,138,509. The formation of silicon carbide (as fibers, films, binders, bulk material and the like) by the energetic treatment of polymeric organosilanes is disclosed, for example, in U.S. Pat. Nos. 4,310,482 and 4,283,376. As U.S. Pat. No. 4,289,720 discloses, multivalent elements such as boron, carbon, nitrogen, oxygen and transition metals and the like, which are incorporated into the silicon-containing polymer, are in general also found in the pyropolymer. Alternatively, such multivalent elements can be incorporated into the final pyropolymer by introducing them subsequent to formation of the initial polysilane (such as during pyrolysis). For example, decomposition of halogenated silanes in the presence of nitrogen sources such as ammonia, nitrogen, or their mixtures to form silicon nitride is reported in assignee's copending U.S. patent application Ser. No. 508,852, filed June 29, 1983, now U.S. Pat. No. 4,505,720. U.S. Pat. No. 4,393,097 describes a similar amorphous silicon-nitrogen-carbon composition formed by chemical vapor deposition. U.S. Pat. No. 4,387,080 describes the heating of a mixture containing silicon, organic silicon polymer, and flaky beta-silicon carbide with gaseous ammonia to give a silicon nitride-containing silicon carbide.